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Change mpi_core_check_sub to be constant time

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Signed-off-by: Waleed Elmelegy <waleed.elmelegy@arm.com>
This commit is contained in:
Waleed Elmelegy 2024-05-08 17:36:09 +00:00
parent e146940714
commit 3235165e07
3 changed files with 39 additions and 2 deletions
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5
library/bignum_core.c
View File

@ -449,9 +449,10 @@ mbedtls_mpi_uint mbedtls_mpi_core_sub(mbedtls_mpi_uint *X,
mbedtls_mpi_uint c = 0;
for (size_t i = 0; i < limbs; i++) {
mbedtls_mpi_uint z = (A[i] < c);
mbedtls_mpi_uint z = mbedtls_ct_mpi_uint_if(mbedtls_ct_uint_lt(A[i], c),
1, 0);
mbedtls_mpi_uint t = A[i] - c;
c = (t < B[i]) + z;
c = mbedtls_ct_mpi_uint_if(mbedtls_ct_uint_lt(t, B[i]), 1, 0) + z;
X[i] = t - B[i];
}

30
tests/suites/test_suite_bignum_core.function
View File

@ -1317,3 +1317,33 @@ exit:
mbedtls_free(X);
}
/* END_CASE */
/* BEGIN_CASE */
void mpi_core_check_sub_ct(char *input_A, char *input_B, int exp_ret)
{
mbedtls_mpi_uint *A = NULL;
mbedtls_mpi_uint *B = NULL;
mbedtls_mpi_uint *X = NULL;
size_t A_limbs, B_limbs;
int ret;
TEST_EQUAL(0, mbedtls_test_read_mpi_core(&A, &A_limbs, input_A));
TEST_EQUAL(0, mbedtls_test_read_mpi_core(&B, &B_limbs, input_B));
TEST_EQUAL(A_limbs, B_limbs);
size_t limbs = A_limbs;
TEST_CALLOC(X, limbs);
TEST_CF_SECRET(A, A_limbs * sizeof(mbedtls_mpi_uint));
TEST_CF_SECRET(B, B_limbs * sizeof(mbedtls_mpi_uint));
ret = mbedtls_mpi_core_sub(X, A, B, limbs);
TEST_EQUAL(ret, exp_ret);
exit:
mbedtls_free(A);
mbedtls_free(B);
mbedtls_free(X);
}
/* END_CASE */

6
tests/suites/test_suite_bignum_core.misc.data
View File

@ -523,3 +523,9 @@ mpi_core_clz:64:0
CLZ: 100000 0: skip overly long input
mpi_core_clz:100000:0
Constant time Subtraction
mpi_core_check_sub_ct:"1234567890abcdef0":"10000000000000000":0
Constant time Subtraction #2
mpi_core_check_sub_ct:"10000000000000000":"1234567890abcdef0":1